Proof-reading
If you are using Microsoft Word to write your assignment you should make sure you use the in-built grammar and spell checker to make basic checks to your essay. You do need to be careful, however, that when you use the spell checker you don't allow Word to make inappropriate changes to things such as author's names or specific terms related to your subject. This can be very annoying for the person marking your work, particularly if Word has changed the name of a well known author/expert within your subject area who you are citing!
Word also cannot detect when a word is spelled correctly but you have used the wrong word for a particular situation.
You may find it helpful to print out your assignment, so that you can proof read it. Often you will pick up more mistakes when you are reading from a printed copy than on screen.
If you are doing your own proof reading a good method is to read your assignment to yourself out loud. Although, doing this might feel strange, reading out loud means that you can examine more closely how your assignment flows and if it makes sense. If you only read your assignment in your head there is a tendency to scan and you are less likely to pick up errors and clumsy sentences.
You can also use the inbuilt text-to-speech tools in Word to have your text read to you. There is guidance on using the Speak tool on the Microsoft site. How well this works for you may depend on the subject area and how many specialist words are included, but is worth a try.
It is also useful if you can get a second opinion, so once you have finished this first stage give your assignment to a constructive person who can also read it for you.
For more guidance on proof reading see the Academic Skills ProofreadingGuide.